rustc should notify users when optimizations are disabled

[tanadeau]

As the Rust community gets more developers coming from JIT'ed or interpreted languages like Java, Python, Ruby, etc., we are going to see more problems where they assume rustc my_file.rs will give fast results even though optimizations are off by default. This can be seen in several threads on Stack Overflow and other forums, including Rust's own (http://users.rust-lang.org/t/reading-numbers-from-a-file/171).

I suggest that when no optimization flags are enabled, rustc should print out a message similar to the following:

rustc my_file.rs
Note: Optimizations are disabled (debug mode). Turn them on by passing the -O flag.

[sinistersnare]

This might be better suited as a Cargo feature, which has text that appears when compiling.

[tanadeau]

@sinistersnare I think it should probably be in both rustc and Cargo, now that you mentioned it. Many beginners are probably not going to use Cargo for their first little hello world-type tests apps, even though they probably should. I know I didn't, especially since the source tarballs with script installers and package managers like Homebrew don't yet include it.

[Valloric]

This would be a good idea. Newbies not compiling with optimizations and then complaining about perf issues is a perennial problem with C++ and it's becoming an issue with Rust as well. Just nip this in the bud with a note.

[sfackler]

OTOH, it seems like it would be annoying for everyone who's not a newbie to be reminded that they don't have optimizations enabled every time they build.

[reem]

I would prefer if this was a cargo feature, that way it would only warn once rather than for every single invocation of rustc.

[Valloric]

OTOH, it seems like it would be annoying for everyone who's not a newbie to be reminded that they don't have optimizations enabled every time they build.

If the user explicitly passes -O0, then the note should not appear.

The idea is that you either know what you're doing, or rustc makes sure that you do.

[tanadeau]

@Valloric Yes. That's exactly the idea I was going for.

[Valloric]

Now that I think about it, I think this issue is going about the problem the wrong way.

IMO the real solution would be to make rustc compile with optimizations by default, and the user must pass -O0 explicitly if they want a debug build. This is the safer approach; if the user forgets a flag, the "failure mode" is they don't have a debug binary when they wanted one instead of them pushing an unoptimized build to production. The first is a nuisance, the second is a disaster (which has bit me in real life more than once).

[tanadeau]

@Valloric I did think of that, but I didn't include that in the issue since there seems to be some strong resistance to "optimize by default" at least within Cargo. See rust-lang/cargo#784 for a somewhat similar discussion there. Turning on optimization by default would definitely be my preference.

[tanadeau]

I should also note that I would be okay (although not enthusiastic) about having this just be a Cargo feature if all of the docs pointed people towards Cargo, which is definitely not the case now. The hello world in the Rust book has people use rustc (http://doc.rust-lang.org/book/hello-world.html).